The multiple causes make it harder to do something about what's called colony collapse disorder, experts say. The disorder has caused as much as one-third of the nation's bees to just disappear each winter since 2006.

Bees, especially honeybees, are needed to pollinate crops.

The federal report, issued Thursday by the Agriculture Department and the Environmental Protection Agency, said the biggest culprit is the parasitic mite varroa destructor, calling it "the single most detrimental pest of honeybees."

The problem has also hit bee colonies in Europe, where regulators are considering a ban on a type of pesticides that some environmental groups blame for the bee collapse. The U.S. report cites pesticides, but near the bottom of the list of factors. And federal officials and researchers advising them said the science doesn't justify a ban of the pesticides yet.

The report is the result of a large conference of scientists that the government brought together last year to figure out what's going on. Participant May Berenbaum, a top bee researcher from the University of Illinois, said the class of chemicals known as neonicotinoids hasn't been proven to be the sole culprit in the bee loss.

In an interview, she said she was "extremely dubious" that banning the chemical would have any effect on bee health. She was the chairwoman of a major National Academy of Sciences study on the loss of pollinators.

Dave Gaulson of the University of Stirling in Scotland, who conducted a study last year that implicated the chemical, said he can't disagree with the overall conclusions of the U.S. government report. However, he said it could have emphasized pesticides more.

Pollinators, like honeybees, are crucial to the U.S. food supply. About $30 billion a year in agriculture depends on their health, said the USDA's Sonny Ramaswamy.

USDA bee researcher Jeff Pettis said modern farming practices that often leave little forage area for bees is a big problem.

At a news conference Thursday with federal officials, Berenbaum said there's no single solution to the bee problem: "We're not really well equipped or even used to fighting on multiple fronts."